For an intelligent battery management, the battery has to communicate with its host device and with its charger. This can be accomplished with the system management bus (SMbus) defined by Intel Corporation in 1995. This system, mainly used in personal computers for low-speed system management communications, is the basis for the smart battery system (SBS).
Specifications of the SMBus can be found in the SBS Implementers Forum (www.sbs-forum.org). Version 1.0 of these specifications was issued on 15 February 1995 and version 2.0 on 3 August 2000. (The Forum is formed by these companies: Duracell, Energizer Power Systems, Fujitsu, Intel, Linear Technology, Maxim Integrated Products, Mitsubishi Electric Semiconductors, PowerSmart, Toshiba Battery, Unitrode, USAR Systems.) The objective behind the SBS was to transfer charge control from the charger to the battery. With an efficient SMBus, the battery becomes a master, tells the charger, acting as a slave, about its chemistry, voltage and size, and thus dictates the algorithm. The battery controls such parameters as voltage, current, switching point, etc.
The term smart battery may include rather different degrees of control. At the very least, the batteries should provide SOC indications. Smarter batteries can control overcharge and overdischarge; even smarter batteries provide more controls and information, this obviously adding to the battery cost. Several companies are now producing more or less sophisticated circuits for smart batteries. They range from the single wire system to the two-wire system of the SMBus.
The single wire system uses only one wire for data communications. In addition to the two battery terminals, another wire (thermistor) allows temperature sensing. This system stores the battery code and provides readings of T, V, I and SOC. It is relatively cheap and finds application in some transceiver radios, camcorders and portable computers. Most single wire systems have different shapes and cannot give standard measurements of SOH. Indeed, these measurements are only allowed when the hosting device is coupled to a designated battery pack. The original battery has always to be used, otherwise incorrect readings are obtained. Using the single wire system in a universal charger (i.e. a charger for all kinds of batteries) is not recommended.
The two-wire SMBus is the most complete system and represents the largest effort to create a standardized communications protocol and smart battery dataset (SBData). In this system, data and clock have separate wires. It can be used in universal chargers, as each battery, regardless of its chemistry, would receive the correct amount of charge, as determined by its specific end-of-charge characteristics.
An SMBus battery contains permanent and temporary data. The former are encoded by the manufacturer and include battery ID code, serial number and type, manufacturer’s name and date of manufacture. Temporary data, that is cycle number, user pattern and maintenance requirements, is acquired during battery usage.
Full SBdata implementation includes (1) execution of all data value functions; (2) meeting the accuracy and precision requirements of all data functions; and (3) maintaining the proper SMBus timing and data transfer protocols.
The data values can be divided into the following categories:
• Historical and identification
• Measurements
• Capacity information
• Time remaining
• Alarms and broadcasts
• Mode, status and errors.
Carrying out measurements at a sampling rate allowing a sufficient accuracy is crucial. The rate needs to be higher when a high current is flowing through the battery.
The SMBus is divided into levels 1, 2 and 3. The first one is not used anymore, as it did not permit chemistry independent charging. Level 2 SMBus allows battery charging within the host (e.g. a laptop computer); the charging circuit may be contained in the battery pack. Level 3 is enclosed in full-featured external chargers. Of course, chargers with level 3 SMBus are sophisticated and expensive. To cut the cost, some chargers with this bus may be not fully SBS compliant. However, in very demanding applications, such as biomedical instruments and precision data collection devices, full compliance is necessary.
Knowing the true state of charge of a battery is not a simple task. Measurements only based on voltage or capacity may prove incorrect, if such factors as temperature, ageing, self-discharge and charge/discharge rate are not taken into account.
Real-time impedance measurements have proven very useful for determining a true SOC, as they eliminate differences in impedance created by battery aging.
A very secure way to authenticate a battery is the so-called random challenge-response authentication.
When the responder, an identification component associated with the battery pack, receives the challenge data, it combines it with a plain-text version of the secret stored in a private secure memory. It performs the authentication transform to calculate the response. On the other side, the host performs the same transform using the same challenge data and the plain-text version of the secret or decrypted secret from the device. The host compares the value that it computes against the response obtained from the identification device. If the calculated data from the authentication component matches the expected answer from the host, then the host authenticates the battery and allows the system to start operation.
An authentication technique of this kind ensures that the replacement battery has the same characteristics of the original one, thus protecting the OEM’s business and ensuring the end-user’s safety and satisfaction.
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06282010
1. Alkaline Batteries
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